Research of Hierarchical P2P Network based on Chord

Haibo Ma, Deguang Wang, Jiamin Zhang, Li Shi Dalian, Liaoning Province, China

Dalian Jiaotong University,

e-mail mhb101@126.com

Abstract: Structured P2P model with Chord method can be used to quickly search the location of resources, which is an effective P2P routing algorithm. This article combined the unstructured and structured P2P model, saying that Hierarchical P2P Network based on Chord as the model of the HPNC. The method is composing the nodes not far from them into a group with unstructured P2P model, the group constitutes the lower net-work; the central node from lower network abstracts into super-node, becomes the upper network node using structured Chord model, the network improves the efficiency of resources search. Building P2P network with HPNC is fast and simple, resources search fast.

Keywords: P2P; chord; DHT; HPNC

1 Introduction In the P2P network, Computing Model calculates from

center to dispersion, from central to the edge, making full

use of the processing capacity of the terminal device,

while each node actively joins in the network to share

resource[1]. P2P network has many kinds of model [2],

like Napster, Gnutella, Chord, CAN, etc. Among which,

the Chord[3] model is representative. Chord, as a typical

and successful routing algorithm of the structured P2P,

has become the hotspot in the present research. The

Chord model uses the ring topology, and its protocol can

map the appointed key words to the corresponding node.

From the aspect of algorithm, Chord is a variant of the

consensus Hash algorithm[4]. However, the Chord to-

pology has a shortcoming that logical overlay network is

completely separated from the topology of the actual un-

derlying Physical layer. In addition, it has neglected the

isomerism between the nodes. In order to enhance the

running and maintenance efficiency of the P2P network,

this paper has improved the P2P model, and putting for-

ward the HPNC (Hierarchical P2P Network based on

Chord).

2 HPNC Network Model Overview 2.1 HPNC topology

HPNC combines unstructured and structured features of

P2P networks. It has two layers, a structured Chord net-

work in upper layer, and a centralized unstructured net-

work in lower layer, and then the integral structure const-

tutes a mixed 2 layers P2P networks.

Nodes are organized in a group according to the MAC

similar first principle. Group is a small unstructured net-

work, in which topology similar to the centralization of

Napster is used, composed of a central node and many

ordinary nodes, while the central node administrates the

ordinary nodes and provides resource index function for

the ordinary nodes in the group. All groups' central nodes

constitute the upper Chord network in the form of super

nodes, and realize accurate and effective Message routing

mechanism based on the DHT[5]. The structure of HPNC

is shown in Figure 1.

(a) (b) Topology of HPNC Nodes classification of

network model lowerlayer

Figure 1. HPNC network model

2.2 Nodes Structure of HPNC

The super node is a great important module of the HPNC

model, charged by the central node in the lower layer,

playing an important part in supporting the route and data

transmission in the entire network. The super node is the

link for the correspondence between the upper and lower

layers, and the interaction between layers is realized by

the super node. The super node, as the node of the upper

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layer Chord network, needs to maintain three forms:

pointer gauge, key value table, and predecessor and suc-

cessor table. The pointer gauge is mainly used between

super nodes in the upper layer for the news routing, and it

will change only when the super nodes add or quit. The

key value table stores all resource information saved on

this super node, and it will vary with the resource infor-

mation being added or deleted. Predecessor and successor

table saves information of the predecessor and successor

nodes of this super node.

Each ordinary node stores the basic information about

the central node in order to communicate with it in time,

so the table is called the central node table. In addition,

each ordinary node has its own shared resources in the

network, which are saved in the resource list.

The central node is a special ordinary node, therefore

firstly it inherits all attributes of the ordinary node.

Moreover, it also needs to maintain two tables for the

group management, respectively the Node_table and the

MessageIndex_table. The former is used to save basic

information of nodes in the group, the latter is used to

save all the resource information issued by the nodes in

the group.

The basic function of the backup node has no differ-

ence from the ordinary node. However, since it also needs

to be the substitute for the central node, besides the ordi-

nary node attributes, it needs to have group internal node

table and predecessor and successor table as the same as

the super node. The backup node needs to periodically

exchange the information of these two tables with the

super node, in order to guarantee the accuracy and time-

liness of the data.

2.3 Features of HPNC

Introduced the nodes grouping thought based on the

physical location, the model will group the nodes accord-

ing to the geographical position in order to group nodes

close in physical space to be in a group. It fully uses the

characteristics of the geographic proximity that the con-

nection and access speed of nodes are fast, the resources

inquiring latency is small, and resources downloading

speed is quick. It has overcome the detour problem of the

structured P2P system that universally exists, and in-

creased the hit rate of the resources localization, reduced

the expenses of the resources localization, and raised the

efficiency of the resources localization.

The model has introduced the concept of the layered

hybrid model. According to different degrees of ability,

nodes are divided into ordinary nodes, central nodes,

backup nodes, and super nodes. The upper layer takes the

central node of the lower layer as the super node to con-

struct Chord structured virtual network, and the lower

layer consists of ordinary nodes and backup nodes, sepa-

rately centralized managed by each group's central node.

This not only fully uses the node computing power and

memory power, but also alleviates huge network fluctua-

tion caused by nodes adding or quitting on the Chord.

By the heartbeat induction mechanism, we can catch

the node invalidation in time and process it rapidly, thus

the fluctuation which the node sudden invalidation

brought to the network will be effectively avoided.

With the new management mechanism and the key

value issue mechanism, and by introducing special node

to manage the network and check the key value issue, the

manageability of the P2P network is strengthened, and the

commercial value of the network is enhanced.

3 Network Maintenance of HPNC 3.1 Node ID Conversation

In the network, each node has its own ID, and different

ID has different function. The node’s behaviors like add-

ing, quitting, or performance updating etc. can make the

node ID converse. Its conversing process is shown in

Figure 2. If the dynamic variation of the nodes cannot be

handled in time, the network fluctuation or even collapse

will be very likely caused. Therefore, effective maintain-

ing nodes' dynamic change, and real-time dealing with ID

conversion between the nodes are necessary means to

guarantee normal operation of the P2P network.

1. The ID conversing process is composed of some ba-

sic behaviors, like the ordinary node adds, exits, fails, and

usurps, the backup node abdicates, fails, and usurps, and

the central node abdicates, fails etc.

2. The ordinary node and the backup node can con-

verse ID, and the backup node and the central node can

switch ID. But the ordinary node and the central node

cannot converse ID directly, the conversation between

them can be completed through transforming to the

backup node.

3. The concept of abdicate and exit must be distin-

guished: Abdicate refers to lose the existing status,

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namely the status degrades, but this node is still in the

network. Such as the backup node abdicates to the ordi-

nary node, although this node no longer has the function

of a backup node, but still exists in the group as an ordi-

nary node. But exit is that the node must quit from the

whole network. The ordinary node can exit directly, but

the backup node must abdicate to an ordinary node first,

then exit the network. In the same way, if the central node

wants to exit, it must abdicates to the backup node first,

then the ordinary node, and then exit.

Figure 2. Nodes ID Conversation

3.2 Group members adjustment

New nodes added to the network may have high per-

formance, so when they join, the central node will call the

group member to adjust function module in order to ar-

range the group member's performance. The node per-

formance is not irrevocable, in different times or certain

circumstances, the node performance will also change

greatly. Therefore, when the node performance parameter

changes, it will inform the central node to update, then

the central node will call the group member to adjust the

module in order to adjust the member ID. When the cen-

tral node finds that an ordinary node’s performance is

higher than the backup one, it will replace the backup

node with the ordinary one. For the same reason, if the

backup node’s performance is higher than the central

node, the two will exchange. Certainly, a new ordinary

node’s performance may extremely superior, so it will be

replaced with a backup node first, then with the central

node. Flow chart is shown in Figure 3.

Figure 3. Adjustments of Group Members

4 Service of HPNC 4.1 Issue Service of Resources Information

The resource information is an information pair which is

composed of the resource key words and IP address of the

issue source.

Any node in network has the resource information is-

sue function. The issue of resource information is com-

posed of introgroup issue and extragroup issue. The in-

trogroup issue is that the central node where the issue

source is adds the resource information to MessageIn-

dex_table for the group member s’ inquiring. Extragroup

issue refers to that the central node converses the resource

information to the identifier (K, N) through the Hash Al-

gorithm, then issues to the super node named

N=Successor(K) according to the Chord principle. For

example, a node whose IP address is 198.10.10.1 issues

resource information that the key word is ring, shown in

Figure 4:

Figure 4. Issue service of Resource Information

The node whose IP address is 198.10.10.1 will be

mapped as node N123 through the Hash Algorithm, if it

acts as an ordinary node in a group which takes node N18

as the central node. If N123 want to issue resource infor-

mation that the key word is ring, firstly N123 transmits

information pair (ring, 198.10.10.1) to N18, then in the

group N18 inserts this information and the issue time into

the MessageIndex_table, like this the resource informa-

tion introgroup issue is completed. Then N18 turns the

information pair to identifier (60, 18, 123) through Hash

Algorithm, while Hash (ring) = 60, 18 is the group mark

where the issue source is, and 123 is the node mark of the

issue source. The K60 successor node is N70, so N18

transmits identifier (60, 18, 123) to N70, and N70 inserts

this data into its own key value table, thus the resources

information extragroup issue is completed.

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4.2 Deletion Service of Resource Information

Any node in network has the resource information dele-

tion function. The resource information deletion is com-

posed of introgroup deletion and extragroup deletion.

According to the node deletion request, introgroup issue

will delete the corresponding information in MessageIn-

dex_table. Extragroup deletion is that the central node

informs the super node which issues that this will delete

the resource information, and lets it delete this resource

information in the key value table.

When some node quits or fails, the central node will

batch delete all information which this node once issued.

4.3 Inquiry Service of Resource Information

Any node that needs to inquire resource must send re-

quest to the central node, and the inquiry request is con-

sisted of key words which the node searches for. The in-

quiry process is mainly divided into three steps. First is to

inquire in MessageIndex_table. If there is no result, Hash

the key words to get the key words’ identifier, then in-

quire it in the key value table. If still not found, search it

in the upper layer through the pointer gauge.

When inquiring in the upper layer, if the key word in-

quired is some K[i].start in the pointer gauge, then return

to its K[i].successor directly. However, the number of

routing information that the pointer gauge saves is only m,

in other words, the key words that the node needs to in-

quire may not in the K[i].start item in the pointer gauge.

In this case, we need to observe which K[i].interval sector

the key words identifier has fallen into in the pointer

gauge. Then forward the inquiry message to the corre-

sponding K[i].successor in this sector, and continue to

search by the K[i].successor node through the pointer

gauge.

Figure 5. Inquiry service of Resources Information

For example, some sub-node Ni of the super node N1

wants to inquire resource that K is 7, but there is no result

in

ly includes two aspects:

f information backup.

the overlay network is how

We all know that the biggest

umber aspect, while the group number is 5, and

localization success ratio of the resource in the group is

50%.

MessageIndex_table and the key value table. There-

fore, N1 searches in its own pointer gauge, and has dis-

covered that K7 is in K[3].interval=[5,1) sector, so it for-

wards the inquiry request to K[3].successor, namely N6

node. N6 searches in its pointer gauge and has discovered

K[1].start=7, the key word that has been searched for, so

it returns the information of N7 which is the successor

node of K7 to Ni. Ni will know the information of the

source issue node of the resource by visiting N7.

5 Performance Assessment of HPNC 5.1 Memory Price of Data

In system memory price main

pointer gauge and the number o

Suppose that Chord and HPNC have the same number of

nodes, expressing the node number in system by Psys-

tem=2N, and group number by Pnumber=2N-M in

HPNC. Suppose that in all situations each group's node

number is the same as Gnumber=2M. Therefore, in the

Chord system the pointer gauge's size which each node

needs to maintain is N, while in the HPNC system, each

super node pointer gauge size is only N-M, and other

nodes do not need to maintain the pointer gauge, obvi-

ously N-M<N. Since HPNC uses the group centralized

management method, each central node needs to maintain

two more tables, Node_table and MessageIndex_table.

Although the system's memory price is increased, more

stable performance and more reliable management are

purchased.

5.2 Searching Efficiency

A key parameter to evaluate

to locate to the nearest node.

problem of the Chord overlay network localization is: In

the logical space, the node relations cannot match the

actual network, namely in the overlay network the

neighboring node is possibly very far in the under layer

physical network. But in HPNC, by the form of MAC

grouping, the information resource is issued inside and

outside the group, the efficiency of searching in physical

local is greatly raised, therefore the problem is well

solved.

Figure 6 is a comparison between HPNC and Chord in

jumping n

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0

1

2

3

4

5

6

7

8

ps

0 10 100 1000 10000 100000

N

Ho

HPNC

Chord

Figure 6. Comparison of searching efficiency performance

5.3 Scalability of System

When the number of nodes in the Chord system increases

me en-

sing number of node.

the number of nodes increases from 210 to 230, dat

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, V

ti and space complexity of the system will expon

tially increase with the increa When

a

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[5] ROWSTRON A,DRUSCHEL P. Pastry: Scalable, distributed

object location and routing for large-scale peer-to-peer systemsstorage capacity of each node will be 3 times. However,

in HPNC, as long as the number of group is invariable,

the data storage capacity will not change a lot. The in-

creasing number of nodes in the group only raises the

data storage capacity of the central node, but will not

cause big fluctuation in the whole network, so it has bet-

ter extendibility.

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